Insectivorous birds are attracted to insect-damaged deciduous trees by plant cues. However, it is unknown whether birds can locate insects on coniferous trees by plant cues induced by insect feeding activity.
Feeding damage may induce plant chemical changes even in parts of a tree where no actual infestation is present. In a laboratory set-up, we studied the behavioural responses of Great Tits (Parus major) and Blue Tits (Cyanistes caeruleus) to Scots pine (Pinus sylvestris) that had been systemically herbivore-induced by pine sawfly larvae (Diprion pini). When birds were simultaneously offered an untreated control Scots pine branch and a systemically herbivore-induced test branch (i.e. a branch without actually feeding larvae) in a study booth, they visited the systemically herbivore-induced test branch first significantly more often
than the control branch. In order to elucidate whether this discrimination was based on visual and/or olfactory plant cues, we compared the light reflectance and odour of test and control branches in a separate experiment. The control branches tended to show higher light reflectance throughout the avian vision range of 300–700 nm than the test branches, but the discrimination threshold model suggests that birds are not able to detect this difference. The systemically herbivore-induced branches emitted the same 29 volatile compounds as control branches, but 21 of them were released in greater quantities by the test branches. Our study shows that herbivore-induced Scots pine branches emit olfactory cues that may allow the birds to discriminate
between uninfested and insect-infested trees, while the role of visual, systemically induced plant cues is less obvious and needs further investigation.